AREA OF THE HEAD, EYES, AND EAR. 739 



most excitable in its middle and posterior part, and becomes less 

 excitable as it is traced towards the prefrontal region. Its action is 

 bilateral, excitation producing a movement of the head and both eyes 

 towards the opposite side (conjugate lateral deviation), which may be 

 simple or may be combined with an upward or downward inclination. 

 Although the head and eye movements are usually obtained together, it 

 sometimes happens that only the eyes are moved, and in other cases 

 only the head as a whole ; the latter appears to be especially the case if 

 the part of the area which is close to the longitudinal fissure or the 

 adjacent portion of the gyrus marginalis is stimulated. 



The area in question has been the subject of special investigation 

 by Mott, in conjunction with the writer. In our experiments 1 we 

 found that in the monkeys which have generally been used for 

 stimulation experiments (Macacus rhesus and Macacus sinicus), it was 

 difficult to obtain any definite results other than the simple lateral 

 conjugate deviation of the head and eyes to the opposite side, together 

 with opening of the eyes and dilatation of the pupil, which had already 

 been described by Ferrier and others. 2 But by using a large species of 

 Cercopithecus, in which this portion of the brain appears to be better 

 displayed than in the macacque, we were able to show that excitation 

 of the upper part of the area usually leads to downward as well as 

 lateral deviation of eyes and head, of the lower part of the area to 

 upward as well as lateral deviation ; whereas excitation of a middle, 

 narrow, wedge-shaped portion produces simple lateral deviation : some- 

 times the downward and upward results were almost unmixed with any 

 lateral deviation (Fig. 337). The greatest effect for all the movements 

 was obtained close to the angle of the precentral sulcus, which may 

 therefore be regarded as the focus of the area. Stimulation of the posterior 

 margin of the area, especially near the focal point, usually also elicits a 

 sharp movement (either pricking or retraction) of the opposite ear or of 

 both ears. The pricking or forward movement is especially caused by 

 stimulation towards the upper part of this margin ; retraction of the ear, 

 by stimulation towards the lower part. 



Risien Russell 3 endeavoured by a special method to differentiate the 

 points on this area in the bonnet monkey, by stimulation after previous 

 division of the lateral recti. He obtained in this "way simple upward and 

 downward movements of both eyes, from points situated close together and 

 near the focus of the area; and by employing the same method in the dog and 

 cat, in which animals stimulation of the head and eyes area only produces 

 normally a simple lateral deviation, he was also able to elicit both an upward 

 movement and a simple downward movement. 



Inhibition effects. — Sherrington has shown 4 that if in the monkey the 

 third and fourth cranial nerves have been severed upon one side, say the left, 

 so that the rectus externus remains the only unparalysed ocular muscle, 



internal capsule and crusta, although not extending down to the pyramids of the medulla 

 oblongata ; and he interprets this to mean that the fibres from the prefrontal region pass 

 to the nuclei of the motor nerves governing the eye movements. But if this were the 

 case the prefrontal region should not be inexcitable. 



1 Mott and Schiller, Brain, London, 1890, vol. xiii. p. 164 ; cf. Pick, Prag. mcd. 

 Wchnschr., 1891. 



2 Ferrier, Proc. Roy. Soc. London,, 1876 ; Horsley and Schafer, Fcstschr. f. 0. Ludwig, 

 Leipzig, 1887 ; and Phil. Trans., London, 1888, B ; Horsley and Beevor, ibid. 



3 Journ. Physiol., Cambridge and London, 1894, vol. xvii. pp. 1 and 378. 



4 Proc. Roy. Soc. London, 1893, vol. liii. pp. 411-420 ; Journ. Physiol., Cambridge and 

 London, 1894, vol. xvii. p. 27. 



